#include<iostream>

using namespace std;

struct ListNode {
    int val;
    ListNode* next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode* next) : val(x), next(next) {}
};

class Solution {
    ListNode* middleNode(ListNode* head)
    {
        ListNode* slow = head, * fast = head;
        while (fast->next && fast->next->next)
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode* mid = slow->next;
        slow->next = nullptr;
        return mid;
    }

    ListNode* merge(ListNode* l1, ListNode* l2)
    {
        ListNode guard;
        ListNode* cur = &guard;
        while (l1 && l2)
        {
            if (l1->val < l2->val)
            {
                cur->next = l1;
                l1 = l1->next;
            }
            else
            {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }
        cur->next = l1 ? l1 : l2;
        return guard.next;
    }
public:
    ListNode* sortList(ListNode* head) {
        if (head == nullptr || head->next == nullptr)
            return head;
        ListNode* head2 = middleNode(head);
        head = sortList(head);
        head2 = sortList(head2);
        return merge(head, head2);
    }
};